1. Field of the Invention
The present invention relates to hydraulic pump assemblies suitable for supplying hydraulic fluid under pressure to a hydraulic control system for example in a vehicle anti-skid apparatus, and more particularly to a hydraulic two-stage pump assembly of the type which includes low and high pressure plunger pumps arranged to be reciprocated by engagement with a single cam shaft.
2. Description of the Prior Art
In a copending U.S. patent application No. 826,617, filed on Feb. 6, 1986, there has been proposed a hydraulic two-stage pump assembly of this kind which includes low and high pressure plunger pumps opposed to each other on a common axis and arranged perpendicularly to a single cam shaft for reciprocation by engagement with the cam shaft. Such a hydraulic two-stage pump assembly is useful to produce sufficient hydraulic pressure while being itself of small volume and compact construction. It has, however, been found that when the low and high pressure plunger pumps are reciprocated respectively to discharge a predetermined amount of fluid under a predetermined pressure, torque of the cam shaft per one rotation fluctuates as shown by a dotted line in FIG. 2. In discharge stroke of the high pressure plunger pump, there occurs a sudden increase in torque of the cam shaft, as shown by the reference character IIa in FIG. 2. This results in undesired vibration of the pump assembly.